In general, an electric double-layer is a structure wherein a thin film layer of an object exhibits continuous presence of positive charges at one side and continuous presence of negative charges at the other side, or distribution of positive and negative charges at the same surface density for two sides, and typically refers to a double layer composed of dipoles. Usually, rearrangement of charges and formation of an electric double-layer occur at the boundary between different materials.
In addition, at the interface between an electrode in the solid state and an aqueous electrolyte solution in the liquid-state, selective adsorption of either cations or anions in the solution or dissociation of solid surface molecules, arrangement/adsorption of dipoles toward the interface and the like may result in formation of electric double-layers. The electric double-layer is also in close connection with a variety of interfacial electrochemical phenomena, such as electrode reaction, electrokinetic phenomena, stability of colloid and the like.
Electric double-layer capacitors (EDLCs) utilizing the electric double-layer are systems that accumulate electrical energy as do batteries, by using the electric double-layer state as a dielectric, via formation of an electrostatic layer on the boundary surface between an activated carbon electrode and an organic electrolyte, and are based on the principle of adsorption/desorption of charges onto the electric double-layers at the interfaces between solid electrodes and solid or liquid electrolytes.
Particularly, the electric double-layer capacitors have a low-energy density as compared to common batteries, but exhibit superior discharge characteristics including instantaneous high current and high output performance, and a feasible semi-permanent life due to charge/discharge cycle characteristics that can be performed several hundreds of thousands times.
Therefore, the electric double-layer capacitors can be suitably used not only as an auxiliary power supply (APS) of mobile information communication equipment, which requires rapid charge/discharge characteristics and high power, such as mobile phones, notebook computers and PDAs, but also as a main or auxiliary power supply of hybrid vehicles requiring high capacity, traffic road safety/guide lamps or blinkers for safe driving of night drivers and uninterruptible power supplies (UPSs).
As active materials for both anodes and cathodes in the electric double-layer capacitors, activated carbon, which has a large surface and is an electrically-stable material, is typically used. Activated carbon has a large surface area of more than 1500 m2/g due to the presence of large numbers of pores on the surface thereof. Even though an aqueous solution or an organic electrolyte may be used as an electrolyte, a size of salt particles should be in the appropriate range in order to obtain large capacity and high charge/discharge characteristics, upon taking into consideration the fact that the pore size of activated carbon is about 2 nm.
In particular, the electrolyte employed in the electric double-layer capacitor is required to meet high adhesivity and low resistance while maintaining high conductivity and capability to dissolve ionic conductive salts at high concentrations.
The electrolytes for the electric double-layer capacitor according to conventional arts employ a solution of tetraalkyl ammonium salts such as tetraethylammonium tetrafluoroborate, dissolved in propylene carbonate or acetonitrile.
Propylene carbonate is widely used as an electrolyte as it is non-toxic and safe and has a high-boiling point. However, propylene carbonate also suffers from limitations in application thereof to large-sized products requiring high output and low resistance, due to its own high resistivity.
For instance, upon performing a life test under high-output conditions of 50 mA/F for the electrolytes utilizing propylene carbonate, the capacity at 20,000 cycle times exhibits a more than 30% decrease, and therefore is measured as a less than 70% level of the initial capacity. Consequently, such propylene carbonate-based electrolytes pose problems associated with difficulty of application thereof to large-scale power supplies of hybrid vehicles or UPSs, requiring constant capacity under conditions of more than 100,000 cycle times.
In addition, acetonitrile is suitable for generating high output due to low viscosity and high solubility in salts, but exhibits various problems such as a low boiling point of 82° C., high inflammability and high probability of cyanide production upon occurrence of a fire. Particularly when it is desired to design large-scale products using acetonitrile, application of heat higher than 140° C. results in sublimation of internal electrolytes which in turn causes the fatal problem of rapid explosion. Further, acetonitrile is a member of organic cyanide compounds classified into categories of toxic substances, and therefore suffers from limitations in use thereof from a standpoint of technical design characteristics attaching great importance to environmental safety.